The present embodiment relates generally to a method of cementing and cementing compositions for sealing a subterranean zone penetrated by a well bore.
In the drilling and completion of an oil or gas well, a cementing composition is often introduced in the well bore for cementing pipe string or casing. In this process, known as “primary cementing,” the cementing composition is pumped into the annular space between the walls of the well bore and the casing. The cementing composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the well bore into subterranean zones. The objective of primary cementing is to prevent the undesirable migration of fluids between such subterranean zones.
Conventional cement compositions, however, mix readily with fluids in the well bore. It has been found that the admixture of as little as two percent of well bore fluid with a cement slurry will contaminate the cement slurry and degrade the competency of the slurry.
Balanced plugs and wells cemented in the presence of oil-based drilling muds (invert muds) are susceptible to well bore fluid contamination during placement. When a cement plug is contaminated with as little as two percent wellbore fluid during placement, it can cause: a slow setting cement plug which results in longer cementing times and increased rig costs, a contaminated cement plug top which results in the need to drill further into the plug to obtain a competent cement top, gas migration, an incompetent cement plug from top to bottom requiring a second plug attempt, and a washed away or diluted cement plug requiring a second plug attempt.
Success is also low with shallow cement plugs which tend to leak and this can be attributed to the cement plugs mixing with mud systems and/or well bore fluids. Invert plug successes also tend to be low and such poor results in most instances can be attributed to the mixing of the cement slurry with the mud system.
Success on invert foam cement jobs is also low due to gas breakout resulting in a poor foam cement job. Gas breakout on such invert foam cement jobs can also be attributed to contamination of the invert foam cement with well bore fluids.
In foam cementing, the admixture of as little as two percent of an oil-based or invert mud will de-stabilize the foam. The de-stabilization of the foam and resultant gas breakout results in higher than desired density thus increasing hydrostatic pressure, gas migration and cement fall back caused by a lower than anticipated cement top.
Therefore, a cementing composition incorporating cement, but having sufficient cohesion to avoid contamination, is desirable for cementing operations.